Two cases of mosaic RhD blood-group phenotypes and paternal isodisomy for chromosome 1

We encountered a 22-year-old man (case 1) and a 23-year-old woman (case 2), both unrelated and healthy. They were mosaic for the Rh blood group phenotype: one erythrocyte population was D-positive and the other was D-negative. Flow cytometric analysis of density profile of RhD antigen in their erythrocytes, and cytogenetic analysis including in situ hybridization using an RHD/RHCE-containing PAC clone, excluded a deletion of the RHD/RHCE gene complex, but suggested the presence of cells with uniparental disomy for chromosome 1 (UPD1). Microsatellite marker analysis was performed in both probands and their family members. In case 1, the analysis with markers spanning the chromosome 1 revealed both maternal and paternal alleles in his peripheral blood leukocytes (PBL), Epstein-Barr virus-transformed lymphoblastoid cells (EBL), and buccal mucosal cells. However, only paternal alleles were detected in all of 50 individual pieces of his hair or hair-roots and all of five monoclonal cell lines cloned from his established EBL. There was no direct evidence of heterozygous, biparental alleles in these two tissues. The presence of maternal isodisomy 1 was not absolutely ruled out in other tissues examined in case 1. Similar results were obtained in case 2, showing biparental, disomic patterns in her PBL and in 15 of 20 pieces of her hair roots, and showing monoallelic patterns in the remaining five pieces of hair roots. Analysis with markers for other autosomes confirmed their biparental inheritance. These findings indicated that both cases had at least two cell populations, one population having paternal UPD1 (isodisomy 1), and another heterozygous, biparental disomy 1. We emphasize that isodisomy for chromosome 1 is not infrequent and may cause unusual RhD phenotype, as seen in cases we described.